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Some body (the Jacks? not sure ) has previously provided their definition of overstuff as being the amount of down to raise the "loose" fabric that spans the distance from one baffle to the next X % above the baffle.

So, normal stuff would be the minimum amount of down needed to fill up the space in a chamber with, for ex., 3" baffles. The down would rise to exactly 3", theoretically any way. So how would that work? 1 oz of 800FP down would fill a 16.34" square chamber to a height of 3", or that same chamber with 3" baffles filled right to the rim? Is that about right?

If there is a somewhat loose piece of fabric, capable of expansion, stretched over that 3" baffle, and enough down is added to rise the "loose" nylon shell up above the baffles by another .6", then that is 20% overfill. Is that right? Anyway, I think that is the definition at least one manufacturer uses.

With that definition, 20% overfill gives you the approximate warmth of 20% more loft. Not quite, because the chamber above the baffles will have max loft in the center and less right above the baffles.

If the chamber is a square above the baffles, with no room for expansion, then what does adding more down beyond the amount needed to fill the chamber do? I'm not sure it would be warmer, as adding more down would replace the air which does the insulating, though it might not decrease warmth until some point. But maybe there are other benefits that would increase warmth? Wind resistance?

Did I also read somewhere that all JRB quilts have at least 20% overfill? If so, Hudson River or Sierra Sniveller 2" baffles + 20%= 2.4" loft(actual rating 2.5). But, MW4 3" baffles with 4" loft , more like 33%. And the MW3 has 2" baffles and 3" loft, so way more "overstuffing there. 50%?

But for one thing, we need to make sure we are all working with the same definition of "overfill". Are we talking about adding more down into a chamber that can not be expanded? Or are we talking about actually increasing loft above the baffles?

If we mean increasing loft above the 2" baffles by 1" or whatever example, then performance should be increase nearly the same as using a quilt not filled beyond it's 3" baffles. So, maybe 15 to 20*F warmer.

I agree that a quilt that is WAY overstuffed will shrink significantly in length and therefore mess up the initial calculation. I was assuming a minor amount of additional fill that wouldn't significantly change the shape of the quilt.

I agree that a quilt that is WAY overstuffed will shrink significantly in length and therefore mess up the initial calculation. I was assuming a minor amount of additional fill that wouldn't significantly change the shape of the quilt.

But wouldn't adding ANY down alter the dimension of the chamber? When empty, those two pieces of 6" wide fabric lie flat and cover six inches. As soon as anything is added to the chamber whether it be down, popcorn or water, the two flat surfaces will begin to form a cylinder just like a platypus bottle is flat until it has water in it. The limit to the thickness is determined by the diameter of the cylinder.

In the example I used, the actual chamber is 453 cubic inches which would require roughly half and ounce of 900 fill down to fully expand in a laboratory setting. JRB uses a 15-22% increase to account for environmental variance. So a 20% overfill for 0.5 ounces would be 0.6 ounces which has the potential to fill 540 cubic inches while the cylinder is actually only 453 cubic inches.
Using more down than that is essentially replacing air pockets with the down, and the air pockets are where the insulating function derives.

Yes the geometry would change but, I'm really talking about warmth.
Warmth does not actually relate to loft, but to volume (to a degree).

Let's take an imaginary example: make a quilt out of rigid material such that is is 2" thick.
Now, fill the quilt with enough down to occupy that volume without being compressed.
That quilt will insulate it's occupant like any 2" quilt that we normally think of.
Next, let's double the amount of down in that quilt.
Now we have a quilt with the equivalent of 4" of down in it, but, since it is rigid, it's still only 2" thick.
BUT, it is as warm as a 4" thick quilt.

There are thick scientific papers on this, the best of which was published by Richard Nisley a few years ago.
But the bottom line is that the insulation comes from hindering both convection and conduction.
As a down quilt is compressed, one losses ground while the other gains ground.
According to Richard's testing, the two properties essentially offset each other until the down is compressed about 2.5 times.

So, if the shape of the quilt can be reasonably controlled, which is why we put baffles in,
the warmth of the quilt is a function of the amount of down used, even if it is compressed a little.

Simply multiply the fill power times the ounces of down then divide that result by the length, and width of the quilt. That will give you the effective loft. Each inch of loft is equivalent to approximately 18*F. Do this for the different amounts of fill that you are considering and you can then see the effect of overfilling.

For practical purposes, don't worry about any compression from over stuffing. You can compress down in half it's normal loft without degrading it's insulating properties.

im a bit confused with this equation, are you multiplying the length and width together before you divide everything else by it, resulting in this equation
(Fill power x Ounces down)/(LxW)
or are you dividing by one and then taking the value and dividing it by the other resulting in this equation
((Fill power x Ounces down)/(L))/(W)
or are you simply adding L&W together resulting in
(Fill power x Ounces down)/(L+W)

Let's take an imaginary example: make a quilt out of rigid material such that is is 2" thick.
Now, fill the quilt with enough down to occupy that volume without being compressed.
That quilt will insulate it's occupant like any 2" quilt that we normally think of.
Next, let's double the amount of down in that quilt.
Now we have a quilt with the equivalent of 4" of down in it, but, since it is rigid, it's still only 2" thick.
BUT, it is as warm as a 4" thick quilt.

So the 2" thick quilt will offer the same thermal resistance as a 4" thick quilt with the same amount of down but half of the entrapped air?

Yes the geometry would change but, I'm really talking about warmth.
Warmth does not actually relate to loft, but to volume (to a degree).

Let's take an imaginary example: make a quilt out of rigid material such that is is 2" thick.
Now, fill the quilt with enough down to occupy that volume without being compressed.
That quilt will insulate it's occupant like any 2" quilt that we normally think of.
Next, let's double the amount of down in that quilt.
Now we have a quilt with the equivalent of 4" of down in it, but, since it is rigid, it's still only 2" thick.
BUT, it is as warm as a 4" thick quilt.

There are thick scientific papers on this, the best of which was published by Richard Nisley a few years ago.
But the bottom line is that the insulation comes from hindering both convection and conduction.
As a down quilt is compressed, one losses ground while the other gains ground.
According to Richard's testing, the two properties essentially offset each other until the down is compressed about 2.5 times.

So, if the shape of the quilt can be reasonably controlled, which is why we put baffles in,
the warmth of the quilt is a function of the amount of down used, even if it is compressed a little.

I suppose that makes sense. However, wouldn't you eventually hit diminishing returns with the shape of the quilt? Where the middle of the chambers are actually touching the hammock, but the overfill is pushing back so hard that it prevents the suspension from snugging the sides of the chambers and the tops of the baffles from actually contacting the hammock? Wouldn't this cause drafts in between the quilt and the hammock along those baffle lines?

Or is down so compressable (even with overfill) that it would still conform to the hammock body?

When ordering a TQ or full length UQ I always have them add 2 extra oz's of down. It is so cheap ($8-$10 bucks per oz) compared to the overall cost of the quilt.
What I think I'm getting is quicker loft, less chance for voids, a little better performance in windy conditions.

im a bit confused with this equation, are you multiplying the length and width together before you divide everything else by it, resulting in this equation
(Fill power x Ounces down)/(LxW)
or are you dividing by one and then taking the value and dividing it by the other resulting in this equation
((Fill power x Ounces down)/(L))/(W)
or are you simply adding L&W together resulting in
(Fill power x Ounces down)/(L+W)

Also, 18*F from what, 100* 75*, assuming you subtract this

The first two options are what I meant - they give identical results.

The 18* per inch comes from WesternMountaineering's tables and the product of the 18xinches is subtracted from 67*, again from WM's tables. WM is one of the most highly respected sleeping bag manufacturers, but they are not cheap (I know, I own two of them).